MANILA (AFP) - The Philippines' police force has sacked 21 officers for being "co-conspirators" in the country's worst massacre six years ago, according to an official statement released on Wednesday (Dec 9).

The officers were dismissed for failing to stop the son of a political warlord and his bodyguards shooting 58 people, including 32 journalists, in the conflict-wracked southern province of Maguindanao in November 2009, according to a police statement.

All but one of the officers were found guilty of grave misconduct.

"The 20 respondents were held liable for opting to become silent spectators to a crime unfolding before their very eyes," said the statement.

"Their inaction manifests complicity and unity of action to those who committed the abduction, and later the murders, themselves."

The remaining officer, Inspector Saudi Matabalao Mokamad, was found to have ignored gunshots he heard from the crime scene and not reporting them to his superiors.

He was sacked on lesser charges.

A lawyer for the victims' families, Harry Roque, told AFP he was "very disappointed" with the pace of the dismissal proceedings.

The officers are among more than 100 suspects on trial for murder before a criminal court, a separate proceeding from the dismissal hearing, and involves members of the influential Ampatuan clan, which allegedly ordered the slaughter in an attempt to crush a rival clan's election challenge.

The brutal massacre, one of the world's deadliest attacks against media workers, saw some shot in their genitals before they were buried in a hilltop grave using an excavator.

The clan's patriarch, Andal Ampatuan Snr, died in hospital last July.

The slow pace of the legal proceedings has angered the families of the victims and frustrated President Benigno Aquino, who took office in 2010 and has promised a verdict before he steps down from office in mid-2016.

Supreme Court Chief Justice Maria Lourdes Sereno said in August the murder trial would finish before the year ends.

Roque said prosecutors would push for a verdict on one of the main defendants, former autonomous Muslim region governor Zaldy Ampatuan, one of the patriarch's other sons, in the first quarter of next year.